Telephone networks implemented with digital facilities can provide both POTS (Plain Old Telephone Service") voiced transmission service and switched 56 kilobits per second (Kbps) data transmission service. The primary difference between the two services is that POTS is primarily used for carrying speech signals and 56 Kbps is primarily used for carrying data signals. Whatever service is selected by a user, it is provided within a single network. For more information on the fundamentals of 56 Kpbs service see A.T.& T. Technical Publication 41458.
Long distance voice calls on POTS telephone circuits require the use of echo cancelers . The echo cancelers remove extraneous echoes that corrupt speech being transmitted over the long distance telephone circuits. However, when switched 56 kilobits per second applications are employed in such POTS telephone circuits, the echo cancelers corrupt the data. As a result, it is desirable for the echo cancelers to be disabled when 56 Kbps service is used. Echo cancelers were originally designed for analog systems and, hence, were designed so that they are disabled upon receiving an analog tone having a frequency of 2100 Hz. With the advent of digitized data systems in which all information was digitized for transmission, the 2100 Hz tone was replicated in digital format by sampling the 2100 Hz analog waveform and generating a stream of digital words, or bytes, representing the wave amplitude at the sampled points. This system works well when the receiver is synchronized with the transmitter, so that each byte is properly framed by the receiver.
Unfortunately, in many instances, the receiver is not synchronized with the transmitter and thus, the echo cancelers cannot be disabled simply by sending a data stream encoding a 2100 Hz tone. A typical environment in which such synchronization is lacking is a 56 Kbps network. In 56 Kbps networks, there are no predetermined byte boundaries; rather data is transmitted as a sequential data stream. As a result, what originates from the 56 Kbps network as a 2100 Hz tone may actually be interpreted as an entirely different tone when it is received and framed at the echo cancelers.
The portion of the telephone network where the echo cancelers reside requires framing of byte boundaries. Since the 2100 Hz tone sent from the remote location lacks byte boundaries, the telephone network attempts to remedy the problem by randomly framing byte boundaries in the sequential data stream. However, since the transmission of the data bits is not synchronized with the framing it cannot be known how the incoming data will be framed as bytes. Because of this inability to know how the data bits will be framed, the transmission of a single version of a digitally encoded 2100 Hz tone is not guaranteed to disable the echo canceler
One means of overcoming the framing problem is to send all seven possible framings (sometimes referred to as "rotations") of the digitized 2100 Hz tone. Seven rotations are certain to cover all the possible framings. A typical example of this approach is discussed in the U.S. Pat. application "METHOD AND APPARATUS FOR TRANSMITTING DATA" Ser. No. 07/210,728 by Schroeder et al. In that patent application, a strategy is disclosed wherein seven digitally encoded rotations of a 2100 Hz tone are stored sequentially in memory. Each rotation is identical to its predecessor except that it is shifted by a single bit relative to its predecessor. The seven rotations are stored in memory corresponding to each possible configuration that each byte of data could assume. These rotations are read out of memory in order and sent to the echo cancelers.
Another approach that accounts for all possible byte framings is disclosed in parent pending U.S. Pat. application, "METHOD AND APPARATUS FOR ECHO CANCELER DISABLEMENT," by Joffe, Ser. No. 07/373,895.
These approaches all send a digitized version of 2100 Hz tone in sufficient bit shifted variations, or rotations, such that at least one such variation will be properly framed at the receiver. This requires use of an undesirable relatively large amount of transmission time. In the former case each rotation is sent for a duration of 0.50 seconds resulting in a total transmission time of 3.5 seconds. In the latter case, each rotation is sent for 0.45 seconds resulting in a total transmission time of 3.15 seconds.